Recently, a semiconductor device including a thin-film transistor (TFT) and a thin-film diode (TFD) on the same substrate and electronic devices with such a semiconductor device have been developed. Such a semiconductor device can be fabricated by making the respective semiconductor layers of the TFT and TFD of the same crystalline semiconductor film on the substrate.
The device characteristics of the TFT and TFD on the same substrate are affected most significantly by the degree of crystallinity of the semiconductor layer to be their active regions. To make a crystalline semiconductor layer of quality on a glass substrate, it is most common to crystalline an amorphous semiconductor film by irradiating it with a laser beam. According to another method, the amorphous semiconductor film may be heated and crystallized after a catalyst element that promotes its crystallization has been added thereto. A third method is to crystallize the amorphous semiconductor film by the latter method and then irradiate the resultant crystalline semiconductor film with a laser beam to further increase its degree of crystallinity. As a result, compared to a normal crystalline semiconductor film that has been crystallized just by being irradiated with a laser beam after having gone through a low-temperature heat treatment process for only a short time, a semiconductor film of better quality, of which the crystallographic plane orientations are aligned to a higher degree, can be obtained.
Patent Document No. 1 discloses an image sensor including, on the same substrate, a photosensor section that uses a TFD and a driver that uses a TFT. According to Patent Document No. 1, the respective semiconductor layers of the TFT and TFD are obtained by crystallizing an amorphous semiconductor film that has been deposited on a substrate.
If the TFT and TFD form integral parts of a single semiconductor device on the same substrate in this manner, not just the overall size of the semiconductor device but also the number of required parts can be reduced, thus cutting down the cost significantly. On top of that, products with new functions, which could not be achieved by conventional combinations of parts, can also be provided.
On the other hand, Patent Document No. 2 discloses a technique for using the same semiconductor film of silicon to form a TFT of crystalline silicon (which will be referred to herein as a “crystalline silicon TFT”) and a TFD of amorphous silicon (which will be referred to herein as an “amorphous silicon TFD”) on the same substrate. Specifically, a catalyst element that promotes the crystallization of amorphous silicon is added to only a portion of the amorphous silicon film on the substrate to be the active region of the TFT. After that, a heat treatment process is carried out, thereby obtaining a silicon film, of which only the portion to be the active region of the TFT has been crystallized but a portion to be the TFD remains amorphous. By using such a silicon film, the crystalline silicon TFT and the amorphous silicon TFD can be fabricated on the same substrate more easily.    Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 6-275808    Patent Document No. 2: Japanese Patent Application Laid-Open Publication No. 6-275807